Why does the paintbrush keep freezing while texture painting?
For painting, you need to be aware of how big your image resolution is and how big your brush is in the view. Close up, your brush is dealing with fewer pixels and will have better response  - further away, it is covering a larger number of pixels to update and will 'freeze' til it calculates each pixel per face.

Freezing can also be due to too much going on to calculate the GLSL feedback , so sometimes it is best to switch to Multitexture in the N panel to focus on just one image texture at a time, clicking the texture Slot that you want active in the Slots tab.